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4.2 Exposure Pathways Pathways of exposure are the means through which an individual may come into contact with a chemical They are determined by environmental conditions (i e , exposed soil or location of groundwater), potential for a chemical to move from one medium to another (Le., soil to groundwater or soil to air), and by general lifestyles of the population Although several potential pathways may exist, usually only a few contribute significantly to the total exposure (Paustenbach, 1990) For a complete pathway to exist, each of the following elements must be present (USEPX 1989) • a source and mechanism for chemical release, • an environmental transport medium (i e., air, water, soil), • a point of potential human contact with the medium, • a route of exposure (t e , inhalation, ingestion, dermal contact), and • systemic bioavailability of the chemical Only complete exposure pathways will be considered in this risk assessment The identification and selection of complete exposure pathways at the Santa Fe Railway Company site are presented in the following section Three potential pathways are considered in this risk assessment These are (1) direct contact with soil containing the TPH, (2) inhalation of hydrocarbon vapors, and (3) migration of TPH to groundwater and potential exposure through groundwater uses 4.3 Evaluation of Potential Exposure Pathways Exposure pathways define the means through which receptor populations may come in contact with chemicals in the environment Exposure pathways are based on the nature and extent of chemicals in environmental media and population location and activity patterns. The relevant exposure pathways are evaluated in the following subsections G WM,M.,W,E FNL\I.W 26 93 4-3 ChmiltwP 190